1. Natural rubber is a polyisoprene biosynthesized in rubber trees. Natural rubber has good elasticity, modulus of elasticity is 2...4MPA, about 1/30000 of steel, and elongation is 300 times that of steel. Natural rubber is a crystalline rubber with high self-reinforcing properties and very good mechanical strength. Natural rubber has good air tightness. Natural rubber has good alkali resistance but is not resistant to strong acid. Natural rubber is a non-polar rubber, so it can only withstand some polar solvents, but expands in non-polar solvents, so its oil resistance and non-polar solvent properties are very poor.
Natural rubber can be used alone in various rubber products due to its comprehensive performance. It can also be used together with other rubbers to improve other rubber properties, such as molding viscosity, tensile strength, etc., thereby improving the performance of rubber products. Widely used in tires, hoses, tapes and various industrial rubber products.
Second, the silicone rubber consists of silicon, oxygen atoms form the main chain, the side chain is a carbon-containing group, and the maximum amount is a silicone rubber with a side chain of ethylene. It is heat-resistant and cold-resistant. It has a temperature of 100...300 ° C. It has excellent weather resistance and ozone resistance as well as good insulation. The disadvantages are low strength, poor tear resistance and poor wear resistance. Silicone rubber is mainly used in the aviation industry, electrical industry, food industry and medical industry.
Third, the molecular characteristics of rubber ... the molecular structure of the rubber elastomer has the following characteristics:
1. A long-chain molecule whose molecule consists of repeating units (links). The molecular chain is soft and its segment has a high degree of activity, and the glass transition temperature (TG) is lower than room temperature;
2. The intermolecular attraction (van der Waals force) is small, amorphous in the normal state (without stress), and the molecules are relatively easy to move relative to each other;
3. Some parts of the molecule can be chemically crosslinked or physically entangled to form a three-dimensional network structure to limit the large mobility of the entire macromolecular chain.
Microscopically, the atoms and segments of the long-chain molecules that make up the rubber are in constant motion due to thermal vibration, causing the entire molecule to exhibit a very irregular random coil shape, and the distance between the two ends of the molecule is much smaller than the length of the straight line. An unstretched rubber image is a tangled mass of curled linear molecules. When the rubber is not subjected to external force, the undeformed state has the largest value. When the rubber is stretched, its molecules are arranged in rows in different directions in the stretching direction. In order to maintain this orientation, it is necessary to work on it, so the rubber is resistant to being stretched. When the external force is removed, the rubber will shrink back to the state where the entropy is the largest. Therefore, the elasticity of rubber is mainly due to the "entropy elasticity" of the change in entropy in the system.
Fourth, the stress of rubber... strain properties
The stress...strain curve is a typical curve of an elongated crystalline rubber whose main component is the entropy change due to the order of the system becoming ordered. With
The molecules are gradually straightened, so that the separation of the branches on the molecular chain disappears, and the intermolecular attraction becomes remarkable, which helps to resist further deformation, so the rubber exhibits high tensile strength when fully stretched. strength.
The stress of a rubber at constant strain is a function of temperature. The stress of the rubber will increase proportionally with increasing temperature.
This dependence of the stress of the rubber on the temperature is called the Joule effect, which accounts for the fundamental difference between the elasticity of the metal and the elasticity of the rubber. In the metal, each atom is held in a strict lattice by the interatomic force. The work done by deforming the metal is used to change the distance between the atoms and cause the change of internal energy. Therefore, its elasticity is called "elasticity." The range of elastic deformation is much smaller than the range of "entropy elasticity" produced in rubber mainly due to changes in entropy in the system. In the general range of use, the stress...strain curve of the rubber is non-linear, so the elastic behavior of the rubber cannot be determined simply by Young's modulus.
Fifth, the relationship between rubber deformation and temperature, deformation speed and time
The deformation motion of rubber molecules cannot be completed instantaneously, because the attraction between molecules must be overcome by the vibrational energy of the atoms. If the temperature is lowered, these vibrations become less active, and the intermolecular attraction cannot be quickly destroyed and thus deformed. slow. At very low temperatures, the vibration energy is not sufficient to overcome the attraction and the rubber becomes a hard solid.
If the temperature is constant and the speed of deformation increases, the same effect as lowering the temperature may occur. In the case of extremely high deformation speeds, rubber molecules do not have time to rearrange, and they appear as hard solids.
Under the action of stress, the molecular chain of the rubber material will be slowly destroyed, resulting in "creep", that is, the deformation is gradually increased. When the deformation force is gradually removed, this "creep" forms a small irreversible shape called "permanent deformation."
Sixth, the electrical properties of rubber
General purpose rubber makes excellent electrical insulation, natural rubber, butyl rubber, ethylene propylene rubber and styrene butadiene rubber have good dielectric properties, so it is widely used in insulated cables. NBR and chloroprene rubber have poor dielectric properties due to the presence of polar or atomic groups in their molecules. On the other hand, the conductive filler such as conductive carbon black or metal powder is added to the rubber to make it have sufficient conductivity to disperse the static charge or even become an electric conductor.
Seven, the thermal properties of rubber
1. Thermal conductivity Rubber is a poor conductor of heat, and its thermal conductivity is about 2.2...6.28 watts/meter 2.0K at a thickness of 25 mm. It is an excellent heat insulating material. If the rubber is made into a microporous or sponge state, the heat insulating effect is further improved, and the thermal conductivity is lowered to 0.4...2.0 watts. Any rubber parts that are in use may generate heat due to hysteresis loss, so care should be taken to dissipate heat.
2. Thermal expansion Due to the large free volume between the rubber molecular chains, the internal rotation of the segments becomes easier as the temperature rises, which increases the volume. The linear expansion coefficient of rubber is about 20 times that of steel. This must be considered in the design of the vulcanization model for rubber products. Because the linear size of the finished rubber product will be 1.2...3.5% smaller than the model. For the same rubber, the hardness and raw rubber content of the rubber compound also have a great influence on the shrinkage rate of the rubber compound. The shrinkage ratio is inversely proportional to the hardness and proportional to the rubber content. The order of the theoretical shrinkage of various rubbers is:
Rubber products should pay special attention to the effect of volume shrinkage when used at low temperatures. For example, oil seals may leak due to shrinkage, and rubber-to-metal bonded products may cause excessive damage due to excessive stress caused by shrinkage.
Eight, the flammability of rubber
Most rubbers have varying degrees of flammability. The rubber containing halogen in the molecule is incorporated into chloroprene rubber, fluororubber, etc., and has certain flame resistance. Therefore, the chlorine-containing chloroprene-containing chlorosulfonated polyene is difficult to burn even after the external flame is removed, and the fluororubber is completely self-extinguishing. The flame retardant can be improved by blending a flame retardant such as a phosphate or a halogen-containing substance into the compound.
